Among needle roller bearings, in which a plurality of needle rollers are arranged along the inner-diameter surface of the outer ring, there are ones using a shell type outer ring formed by pressing including a drawing step. Applications of shell type needle bearings using such a shell type outer ring range extensively because of their economical advantages. Recently, applications in which long lives are required are increasing.
Schematic steps of pressing of a conventional shell type outer ring are as follows. First, a circular blank is formed into a cup shape in a drawing step, and the corner portion of the cup bottom is restruck to a predetermined corner radius in a restriking step. Thereafter, the central portion of the cup bottom is punched in a bottom punching step to form one of the flanges of the outer ring, and the top end portion of the cup is trimmed to a uniform height in a trimming step. After the drawing or restriking step, an ironing step may be added. Ordinarily, such pressing is carried out using a transfer press or a progressive press. If a transfer press is used, a punching step of a circular blank is often incorporated together. The other flange of the outer ring is formed by bending the top end of the cup inwardly in an assembling step after heat treatment.
For the blank material for such a shell type outer ring, a steel plate of a casehardened steel such as SCM415 is used. In order to ensure predetermined product strength, it is subjected to heat treatment such as carburizing/hardening or annealing after pressing. Since a steel plate of a casehardened steel is high in the carbon content compared with a soft steel plate such as SPCC and thus the r value, which is a standard for drawing properties, is low, the number of drawings in the drawing step is divided into a plurality of times to set the drawing ratio per drawing small.
As described, since a shell type outer ring is formed through many pressing steps, due to accumulation of precision errors of molds and uneven strains in each working steps, it is inferior to an outer ring formed by cutting in the roundness of the tubular portion and the dimensional accuracy of the amount of uneven thickness, so that the life of the bearing is short. As one in which heat treatment of a shell type outer ring is carried out after the assembling of the bearing, and in which the outer ring is further hardened and tempered after carburizing/nitriding in order to improve the life of such a shell type needle roller bearing, there is a manufacturing method of a shell-type needle roller bearing in which the outer-diameter roundness of the outer ring is increased and the strength of each bearing part is increased (For example, see JP patent publication 3073937, pages 1-2, FIGS. 1-3).
On the other hand, among compressors for e.g. air-conditioners, there are ones using a support structure in which compression action members are actuated by driving the spindle, and radial loads on the spindle are supported by a needle roller bearing arranged in the compressor (For example, see JP patent publication 2997047, page 2, FIGS. 10-12). A needle roller bearing has an advantage that in spite of the fact that the projected area is small, a high load capacity and a high rigidity can be obtained. Thus, a support structure for a compressor spindle can be designed compact.
In a needle roller bearing employed for such a support structure for a compressor spindle, since a lubricating state tends to be thin due to mixing of a coolant, and also, the spindle rotates at a high speed, surface damage such as smearing or surface starting type peeling may develop on the inner-diameter surface of the outer ring, on which the needle rollers roll, thus shortening the bearing life. Also, in a compressor for a vehicle air-conditioner, it is required to reduce noise during use of the bearing due to the rolling of the needle rollers.
Also, in an automatic brake system such as an anti-lock brake system (ABS) or traction control (TRC), a piston pump for feeding brake fluid in a reservoir tank into a master cylinder is provided. Among piston pumps for feeding oil or the like under pressure, there are ones in which an eccentric portion is provided on an armature shaft which is the output shaft of the electric motor, and the reciprocated piston is abutment-supported by a rolling bearing fitted on the eccentric portion (For example, see JP patent publication 8-182254, page 2 and FIG. 7). There are also ones in which a needle roller bearing is employed for a rolling bearing for abutment-supporting such a piston (For example, see JP patent publication 2001-187915, page 2 and FIG. 9).
In a needle roller bearing employed in a support structure of the piston pump driving portion, since the lubricating state becomes thin due e.g. to mixing of low-viscosity oil (brake fluid), and also, the needle rollers roll at a high speed on the inner-diameter surface of the outer ring, which the piston abuts, a runout of oil film tends to occur on this rolling surface. Thus, surface damage such as smearing or surface-starting type peeling may occur on the inner-diameter surface of the outer ring, thus shortening the bearing life. In a piston pump mounted in a vehicle brake system such as ABS or TRC, too, it is required to reduce the noise during use of the bearing due to the rolling of the needle rollers.